egl: add EGL_IMG_context_priority to the whitelist

In the CCD 7.0 document EGL_IMG_context_priority was added as an
mandatory egl extension devices must support. However, right now the
extension string is not returned to applications even if the graphics
driver supports it.

Fix this by adding the extension to the whitelist.

Test: check for the extension string returned by eglQueryString

Change-Id: I7833a4e1782e12d845207b234431e10e09c085dc
Signed-off-by: Christian Poetzsch <christian.potzsch@imgtec.com>